1. Field of the Invention
The present invention is directed to an improved process for preparing alkyl and aryl substituted oligosiloxanes suitable for use as diffusion pump oils. More particularly, the present invention is directed to employing Grignard reactions for the preparation of 1,3,5-trimethyl, 1,1,3,5,5-pentaphenyltrisiloxane which is eminently suited for use as a diffusion pump oil.
2. Brief Description of the Prior Art
Organic liquids or oils have been in use for a long time as diffusion pump oils. In order to be suitable for use as a diffusion pump oil, an oil must have low volatility and yet be vacuum distillable. Perhaps even more importantly, the oil must have a very high degree of thermal stability.
It was recognized a long time ago in the prior art that certain organo-siloxane compounds readily meet the above-noted requirements and therefore are well suited for use as diffusion pump oils. More particularly, it was recognized in the prior art that certain methyl and phenyl substituted trisiloxanes serve well as diffusion pump oils. Still more particularly, British Pat. No. 787,800 describes the synthesis and use of 1,3,3,5-tetramethyl,-1,1,5,5-tetraphenyltrisiloxane as a diffusion pump oil. British Pat. No. 808,613 describes the preparation and use of 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane as a diffusion pump oil.
Although the application of the above-noted specific compounds as diffusion pump oils has been sustantially universally accepted in the art, the use of these compounds still suffers from the disadvantage of relatively high costs. The high costs are principally occasioned by the general inadequacy, and more particularly by the low yields of the state-of-the-art synthetic processes for preparing these compounds.
For example, 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane has hitherto been prepared by co-hydrolysis of the corresponding alkyl and aryl substituted chlorosilanes, by catalytic co-polymerization of tetraphenyldimethyldisiloxane and phenylmethylsiloxane, or by the reaction of the sodium salt of diphenylmethylsilanol with phenylmethyldichlorosilane. British Pat. No. 808,613 states that the "best" process for preparing 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane involves the reaction of diphenylmethylsilanol with phenylmethyldichlorosilane. As is stated in British Pat. No. 787,800, the "best" process for preparing 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane involves the reaction of diphenylmethylsilanol with dimethyldichlorosilane.
Because experience has shown that the above-noted two processes are relatively uneconomical due to their low yield and excessive side product formation, the principal object of the present invention was to provide an alternative synthetic route to alkyl and aryl substituted oligosiloxanes in general, and to 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane in particular. The desired synthetic route was expected to be more economical principally because of lesser degree of side product formation and hence better yields.
The present invention utilizes aryl Grignard reagents to replace alkoxy leaving groups either in already formed alkyl, alkoxy substituted oligosiloxanes or in alkyl-alkoxy substituted silane precursors of the desired oligosiloxanes. In order to properly illuminate the background chemistry of the present invention, attention is directed to the book "Chemistry and Technology of Silicones", written by Walter Noll, Academic Press, New York, San Francisco, London, 1968 and particularly to pages 41-49 thereof. The above-noted pages discuss the use of organometallic reagents, such as Grignard reagents in the chemistry of silicones. Other references of interest to the present invention are the following: "Organosilicon Compounds" by C. Eaborn, Butterworth Scientific Publications, 1960 (London), pages 307-312, and "The Partial Hydrolysis of Methyltrimethoxysilane" by M. M. Sprung and F. O. Guenther, J. Am. Chem. Soc., 77, 4173 (1955).